Crayfish and flatworms coevolved, but now face coextinction

DNA sequencing by Museum scientists has revealed how endangered Australian crayfish and their symbiotic flatworms evolved together - and may soon become extinct together too.

The research was part of an international collaboration led by University of Cambridge scientist Dr Jennifer Hoyal Cuthill, Museum team Dr Tim Littlewood and Dr Peter Olson, and senior author Dr David Blair of James Cook University.

The scientists worked together to help reconstruct the evolutionary history of mountain spiny crayfish (Euastacus species) and the flatworms (Temnocephalida) that live on them.

They found that the two groups have evolved and diversified together for at least 80 million years, since the Cretaceous Period.

According to the research, this strong association means that current ecological threats to the crayfish could also lead to the extinction of the flatworms.

'The intimate relationship between the crayfish and flatworms is unique and long-lived,' says Dr Littlewood. 'Not just during the lifespan of the individual organisms themselves, but throughout their evolutionary history.

'Yet with 75% of Euastacus crayfish species either endangered or critically endangered, we could soon see many of the flatworms vanish alongside them.

'Untangling this relationship gives us a clear insight into their past climates and environments, and highlights the current threats to their biodiversity.'

Habitat loss

Mountain spiny crayfish are found across eastern Australia, and live in cool, freshwater streams. Many of them play host to temnocephalan flatworms, forming a symbiotic relationship that both parties benefit from.

These temnocephalans live either on the surface of the crayfish, where they catch food that floats by, or inside the gill chamber, where they can feed on parasites.

Yet relying on one another too much has its risks. As the Australian continent has drifted northwards over the last 165 million years, significant ecological changes have fragmented crayfish habitats, confining them to ever-smaller areas.

As a result, mountain spiny crayfish are now severely threatened by climate change, with 75% of species classified as either endangered or critically endangered. This is most severe in the north of the range in Queensland, where the climate is hottest.

This habitat loss has had a knock-on effect on the flatworms. By analysing the DNA of 33 species of flatworm, the scientists found that instead of adapting to hosts beyond the crayfish, the flatworms continued to evolve alongside them. As they grew more isolated, only relying on each other, both groups became more specialised and genetically distinct - leaving them vulnerable to coextinction.

Partnership in peril

To quantify this vulnerability, the team used computer simulations that accounted for the close evolutionary relationship between the two groups.

These showed that if all the mountain spiny crayfish currently endangered were to die out, 60% of their flatworm symbionts would also be lost.

As the most isolated populations in both groups tend to be the most genetically distinct, the species at greatest risk of coextinction are also the most evolutionarily distinctive, which makes the loss all the more unfortunate.

'This ancient partnership is now at risk,' says Dr Hoyal Cuthill. 'These species represent a long history of coevolution, but they're likely to suffer coextinction if they are not protected from further environmental and climate change.

'This study highlights the threats that climate change poses to ecosystems worldwide, and we hope it will lead to more research into the effects of extinction on other symbiotic species.'

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